Device for determining the effectiveness of stimulation in an electrical heart stimulator

Abstract

A device for determining the effectiveness of the electrical stimulation of heart muscle from a signal having a post-potential component and, with effective stimulation, a superimposed evoked response component. The device includes a differential stage with a first input for application of the signal and a second input for application of a feedback signal. The differential stage generates a corresponding output signal whose level is determined by the levels of the signals present at the first and said second inputs. A comparator stage has feedback units able to act on the second input in a follower relationship with respect to the signal present at the first input avoiding saturation of the differential stage. The feedback units generate at least one compensation signal indicating the variation in the signal present at the first input over time. The compensation signal is indicative of the evoked response superimposed on the post-potential component.

What is claimed is:  
     
       1. A device for determining the effectiveness of electrical stimulation of heart muscle from a signal comprising a post-potential component having, in the event of effective stimulation, a superimposed evoked response component, the device comprising: 
       a differential stage with a first input for application of the signal and a second input for application of a feedback signal, the differential stage generating a corresponding output signal whose level is determined by the levels of the signals present at the first and said second inputs; and  
       a comparator stage including a feedback unit configured to act on the second input in a follower relationship to the signal present at the first input avoiding saturation of the differential stage;  
       the feedback unit being configured to generate at least one compensating signal indicative of the variation of the signal present at the first input over time, the at least one compensating signal being indicative of the presence of the evoked response.  
     
     
       2. The device according to  claim 1 , wherein the comparator stage comprises at least two threshold levels which when reached by the output signal from the differential stage are indicative of possible saturation of the differential stage relative to its linear functioning dynamics. 
     
     
       3. The device according to  claim 2 , wherein the comparator stage comprises at least one further threshold defining a field of values for the output signal from the differential stage in which the feedback unit is substantially inactive. 
     
     
       4. The device according to  claim 1 , wherein the feedback unit is configured to generate first and second compensation signals which are indicative, respectively, of the divergence between the output signal from the differential stage in a first and a second direction, respectively, with respect to a selected reference level. 
     
     
       5. The device according to  claim 4 , wherein the feedback unit is configured to apply a signal obtained from the sum of the first and the second compensation signals to the second input of the differential stage. 
     
     
       6. The device according to  claim 5 , wherein the sum is obtained with different signs for the first and said second compensation signals, respectively. 
     
     
       7. The device according to  claim 1 , wherein the feedback unit comprises an integrator stage configured to generate a signal applied to the second input by integration. 
     
     
       8. The device according to  claim 1 , wherein the at least one compensating signal is a pulsed signal in which the frequency of the pulses is indicative of the difference between the output signal from the differential stage and a selected reference level. 
     
     
       9. The device according to  claim 1 , wherein the comparator stage is configured to generate first and second compensation signals of the pulsed type, in which the pulse frequency is indicative of the derivative of the signal present at the first input with respect to time, the first and second compensation signals being generated alternately between them according to the sign of the derivative. 
     
     
       10. The device according to  claim 9 , further comprising a counter and wherein the first and second compensation signals are input respectively as increasing and decreasing signals to the counter, the progression of the count in the counter over a selected period of time comprising a sequence of count signal values indicative of the effectiveness of the stimulation pulse. 
     
     
       11. The device according to  claim 10 , wherein the counter is configured so as to be zeroed corresponding to the action of electrically stimulating the heart muscle. 
     
     
       12. The device according to  claim 11 , wherein the counter is enabled after a predetermined time interval following stimulation of the heart muscle. 
     
     
       13. The device according to  claim 10 , wherein the counter is configured to be enabled for the purposes of performing the corresponding count during a time window of a size determined from the stimulating effect. 
     
     
       14. The device according to  claim 13 , wherein the time window has a duration ranging from 50 to 60 milliseconds. 
     
     
       15. The device according to  claim 10 , further comprising a processing logic module which is capable of applying to the count signal values from the counter during the selected period of time at least one criteria for identifying the effectiveness of stimulation selected from the group of: 
       1) whether the sum of all the negative values which are greater in absolute value than a selected threshold value exceeds a predetermined limit,  
       2) whether the maximum value of the count signal values, in modulus and sign, is greater than the first value of the count signal values incremented by a specified amount, and  
       3) whether reduction of the sequence of the count signal values by interpolation into a series of segments of a straight line identified by their corresponding angular coefficients, with subsequent comparison of the angular coefficients with corresponding pairs of selected limit values results in at least one of the angular coefficients exceeding the corresponding pair of limit values.  
     
     
       16. The device according to  claim 15 , wherein the processing logic module is configured to generate an output signal indicative of effective stimulation when an affirmative result is obtained from one of the criteria. 
     
     
       17. The device according to  claim 16 , wherein the processing logic module is configured to apply the criteria in sequence, passing on to the next criterion if a negative result is obtained from one of the criteria. 
     
     
       18. The device according to  claim 15 , wherein the processing logic module is configured to generate an output signal indicative of ineffective stimulation when all three of the criteria yield a negative outcome. 
     
     
       19. The device according to  claim 14 , further comprising means for detecting excursion of the count signal values during the selective period of time between a selected maximum and a selected minimum value and for declaring that the stimulating action is ineffective if the detected excursion is less than a predetermined limit. 
     
     
       20. The device according to  claim 15 , wherein the processing logic module is configured to detect a difference between a maximum value and a minimum value of the sequence of count signal values subjected to the third criterion and to apply the third criterion only if the difference between the maximum value and the minimum value is greater than a predetermined limit. 
     
     
       21. The device according to  claim 20 , wherein the processing logic module is configured to avoid application of the third criterion when the difference between the maximum value and the minimum value is less than the predetermined limit. 
     
     
       22. The device according to  claim 15 , further comprising at least one processing logic module selected from: 
       a filtering module to reduce the spectral content of the signal transferred to the logic module at higher frequencies,  
       a module to differentiate the sequence of values subjected to the filtering, and  
       a module to translate the values obtained from this differentiation in such a way that the last of them is always zero.